Thursday, March 3, 2011

【 Weak current College 】 for Zoom lines of analysis method 】

"Electronics" is an important electronic professional technical courses. Analog circuits are difficult to learn, teachers to teach a course. Amplifier is analog circuit basics, but also the electronic technology of focus and difficulties that only solves the problem, in order to enter the field of electronic technology. The author has been teaching practice, and gradually formed a nonlinear devices, linear devices and linear amplification of contradiction to linear, nonlinear and linear as the main line, DC analysis and exchange of analysis as the main content of the analysis of the amplifier and the principle of better solved from the circuit of the electronic technology of transition, a solution to getting started with electronic technology difficult issues.

1 from linear to nonlinear

Electronic circuit is a circuit of one branch, is contained in circuits, electronic components and electronic devices are nonlinear devices, electronic circuit is nonlinear circuit. The circuit of nonlinear circuits in General including a chapter, but the content, just simple introduction, not attract enough attention to students. So the electronic technology of course well from the outset to nonlinear circuits linear circuits of transition.

The electronics of the start telling the pn junction, pn junction is the basis of semiconductor devices. In discussing the PN works, get the current-voltage characteristics of pn junction, entered the nonlinear characteristic curve of the VA to nonlinear function. Here first of all to give a definition of linear resistance leads DC (static) resistance and Exchange (dynamic) comparison of concepts of resistance, linear resistance (v-I characteristic curve is through the origin of a line, the DC resistance and AC resistance equal and as a constant) can be drawn from the following important conclusions:

(1) nonlinear components in the current-voltage characteristics curve arbitrary point of DC resistance and AC resistance in General is not equal.

(2) nonlinear components of DC resistance and AC resistance is not a constant, but rather as the quiescent point.

Pn junction is the resistance is small, but quite the reverse resistance. So, often to the electrical conductivity of nonlinear summarized as one-way. Diode is a pn junction, transistor by PN, when he worked in zoom State input characteristics is equal to PN forward, while output characteristics is equal to the base area into minority carrier under the control of the pn junction reverse characteristics.

The above story is nonlinear, electronic components are electronic components-linear characteristics of the circuit and the difference between the General linear circuits, to understand the amplifier works, the quiescent point settings and DC analysis and analysis of the different exchanges.

2 Zoom line of nonlinear characteristics of the

Nonlinear components often yields new frequency components, also is to produce non-linear distortion. This is the electronic circuit, the primary issues must be considered. If the AC signal directly to the transistor's launch knot (that is, not a static offset), since the launch of one-way conductivity, even ignored his dead voltage and positive characteristics of nonlinear, it will give rise to serious nonlinear distortion, so that only positive half week breakover, negative half week is due (CPI working condition). Only AC signal voltage of central location along the axis of the up pan, that is, at the launch of Canada forward bias, and so that the forward bias value is greater than the AC signal amplitude of the pn junction in AC signal of positive and negative half week are conduction (group a State) that can be enlarged without distortion, the resulting 2 conclusions:

(1) in order to overcome the PN one-way conductivity of nonlinear distortion, amplifier before joining AC signal must be coupled with the DC bias signal.

(2) of the amplifier circuit is a DC signal and communication signal; 2 signal circulation loop may be different, that is, both direct access and Exchange; amplifiers all voltage and current for both DC and AC component, that is, the amount equal to the direct flow of instantaneous exchange amount, which meant amplifier analysis includes DC analysis and exchange of 2 parts, DC analysis is to determine the work point of DC, AC analysis is calculation magnification, input and output resistors, power output and efficiency, as well as the frequency response and other performance indicators. DC signal and communication signal pathways, especially nonlinear devices on DC signal and communication signals the performance difference (DC resistance and AC resistance), DC analysis and exchange of analysis you want to use a different circuit network and parameters. These are often ignored by some students, particularly attracted attention.

3 weak signal equivalent circuit analysis of linear

The weak signal analysis method of the amplifier, put him into a linear circuit analysis, this completes the linear-linear-linear. But this is not a return to its original place, but there is a qualitative leap. Although the Zoom line AC analysis is linear analysis, non-linear devices must be used in a given static work point of communication parameters, nonlinear characteristics here still play a role in many electronic circuit, is the use of electronic devices of DC resistance and AC resistance of a different nature. Take advantage of this concept can be understood and interpreted many circuit works. For example, active load is the use of this feature, in the low voltage DC power supply or greater static operating current, a greater exchange of equivalent resistance, differential amplification circuit, through the long tail differential amplifier emitter resistors feature analysis, knows that he can effectively reduce the magnification and the on-mode differential-mode signal has no effect, so he's better, if you use linear resistance in certain operating current, choosing great resistance, it must be the emitter voltage DC power supply, so that the selection of AC resistance is very large and very small DC resistance of active load is very natural.
4 closing
The circuit of course general linear circuit analysis, to electronic devices in non-linear, then to zoom line AC signal linear analysis, at this point, the Zoom line analysis task has basically completed,The circuit is a circuit components of the circuit, the analysis of the circuit is the first circuit component model to replace these components, use circuits basic laws and basic analysis method, on the circuit elements of model components of the network solution, electronic circuits and the only difference between General circuit lies he contains electronic devices. Now, when the electronic devices also used his circuit model instead, electronic circuit also becomes a general analysis of the circuit, it also becomes a general analysis of the circuit, so to speak, is to make students understand the electronic circuit is a circuit of a branch, the electronics "is the continuation of the circuit and the expansion of its basic laws and analytical methods are the same, thereby establishing a unified, integrated circuit analysis concepts.

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